<div style="
   position: absolute;
   top: 0px;
   left: 360px;
   min-width: 160px;
   height: 34px;
   overflow: hidden;
   background: #F2F2F2;
       -moz-border-radius:10px;
    -webkit-border-radius:10px;
    border-radius:2px;">
<select id="tema_id" onchange="botonSiguiente()" style="background: transparent;
   min-width: 160px;
   padding: 5px;
   font-size: 16px;
   border: 1px solid #ccc;
   height: 34px;
">
    <option value=""></option>
    <?php foreach($temas as $tema): ?>
        <option value="<?php echo $tema->getId();?>"><?php echo $tema->getTitulo(); ?></option>
    <?php endforeach; ?>
</select>
</div>

<span id="span_boton">
    <a class="botonSeleccionarTema" onclick="seleccionarVideo()">Siguiente</a>
</span>


<script>
    function botonSiguiente(){
        var tema_s = $("#tema_id").val();
        if(tema_s !=""){
            $("#span_boton").show();
        }
        else{
            $("#span_boton").hide();
        }
    }
</script>